It is generally considered desirable to reduce the level of noise within a vehicle passenger compartment. External noises, such as road noise, engine noise, vibrations, etc., as well as noises emanating from within passenger compartments, may be attenuated through the use of various acoustical materials. Accordingly, sound attenuating materials for vehicles, such as automobiles, are conventionally used in dashboards, in conjunction with carpeting for floor panels, in wheel wells, in trunk compartments, under hoods, as part of headliners, etc.
The attenuation of external noise (i.e., noise emanating from a source external to a vehicle compartment) is conventionally referred to as sound transmission loss (STL). The attenuation of internal noise (i.e., noise emanating from within a vehicle compartment) is conventionally referred to as sound absorption. The acoustic impedance of a material is defined as material density times acoustic velocity, and is expressed in units of Rayls (Newton−seconds/meter.sup.3). Acoustic impedance defines how easy it is for air to move through a material. Thus, for fibrous materials, acoustic impedance may depend upon the density of the fibrous material and fiber diameter. Generally, the heavier the blanket and the finer the fibers, the higher the acoustic impedance. Moreover, thicker layers typically have more acoustic impedance than thin layers. The ability of a material to attenuate noise is conventionally defined by the material's collective STL, acoustic impedance, and absorption characteristics.
Carpeting for use within vehicles is conventionally tufted or nonwoven. Tufted carpet generally includes a composite structure in which tufts, or bundles of carpet fibers are introduced (such as by stitching) into a primary backing, such as a woven or non-woven fabric. A secondary backing or coating of thermoplastic material is then applied to the underside of the carpet construction in order to securely retain the tufted material in the primary backing. This secondary backing not only dimensionally stabilizes the construction but may also provide greater abrasion and wear resistance, and may serve as an adhesive for an additional layer of material. Nonwoven carpet is composed of fiber that is mechanically entangled by needling, water jet, or other processes.
Vehicle carpeting is conventionally molded into a non-planar three dimensional contoured configuration which conforms to the contours of a vehicle floor. A secondary coating of thermoplastic material applied to the primary backing may also make the carpeting moldable and shape-retaining, and may also serve as a barrier to improve the sound attenuating properties of the carpeting.
Conventional vehicle carpeting typically includes an ethylene-vinylacetate (EVA), polyethylene (PE), or polyvinylchloride (PVC) layer which serves as a barrier sheet. Unfortunately, there are several drawbacks associated with the use of EVA, PE, and PVC layers in these vehicle applications. For example, EVA, PE, and PVC are non-porous materials which can be relatively heavy when applied to carpeting, dash insulators, and other interior trim components.
Various sound attenuating materials have been developed for use in reducing noise levels within passenger compartments of vehicles. For example, U.S. Pat. No. 4,851,283 to Holtrop et al., proposes a thermoformable laminate for use in headliners. The headliner comprises a non-woven fabric bonded to a foamed polymer sheet. The fabric is formed from a blend of low melting staple fibers and high melting staple fibers.
U.S. Pat. No. 5,298,694 to Thompson proposes a non-woven acoustical insulation web. The web comprises thermoplastic fibers, and particularly a blend of melt-blown microfibers and crimped bulking fibers.
U.S. Pat. No. 5,677,027 to Masuda et al. proposes a sound insulating structure comprising a covering layer, a panel, and a cushioning layer. The cushioning layer comprises a first fiber such as polyethylene terephthalate (PET) and a second fiber that is of a shell-core construction wherein the majority of the core is PET.
U.S. Pat. No. 5,817,408 to Orimo et al., proposes a sound insulating structure which includes low and high density thermoplastic fibers. PET is preferred as a thermoplastic synthetic fiber.
U.S. Pat. No. 4,529,639 to Peoples, Jr. et al. proposes a molded foam-backed carpet assembly which includes a carpet layer, a moldable thermoplastic polymer layer and one or more foam pads fusibly bonded to the thermoplastic layer and extending over less than the entire surface of the thermoplastic polymer layer to provide desired cushioning and sound and thermal insulation only in preselected areas of the carpet.
In general, the ability of conventional vehicle carpeting to attenuate sound increases as the amount of material increases. Unfortunately, increased carpeting material often increases the weight as well as the cost of vehicle carpeting, which is typically undesirable. Accordingly, there is a continuing need for carpet materials for use within vehicles that exhibit good sound attenuating properties, that are also lightweight and low in cost and which may be tailored to conform to local geometry and acoustic requirements. This tailoring may include the use of multiple materials disposed as multiple layers some of which may be sound absorbing and others acting as a barrier. The layers may further be provided only in local areas to match the location of the source of the noise or may cover the entire carpet backing layer.